A Search for Far-Ultraviolet Emission Lines from Diffuse Hot Gas in the Halo of NGC 4631

Abstract

During the Astro-2 mission in 1995 March, the Hopkins Ultraviolet Telescope (HUT) was used to conduct a sensitive search for far-UV emission lines from hot gas in the halo of NGC 4631. This galaxy has strong radio continuum emission, extended Hα emission, and soft X-ray emission from an extended halo. Recent ROSAT observations indicate that much of the X-ray emission comes from a very soft component, with a temperature less than 6 × 105 K. Gas cooling through this temperature range should show emission in the prominent O VI λλ1032, 1038 and C IV λλ1548, 1551 lines. Hot gas interacting with cold clouds at the interface of the disk and the halo may form turbulent mixing layers, which should produce strong emission in lower ionization species such as C II λ1335 and O III] λλ1661, 1666. The aperture of the HUT spectrograph subtends 10'' × 56'' and was positioned parallel to the disk 39'' (1.4 kpc) south of the nucleus. No emission lines were detected. The derived constraints are tempered somewhat by the unknown extinction within the NGC 4631 halo. If it is low, the upper limits suggest that the mean mixing-layer temperatures are less than 2 × 105 K or that mixing layers are not a significant contributor to the observed Hα emission at the position of the HUT slit. For smooth flows, the upper limit to the O VI flux suggests that less than approximately 15 M☉ yr-1 are processed through a galactic fountain in NGC 4631.